3. And there is a great difference amongst the shells themselves, for some are smooth, as the solen, mya, and some conchæ, called by some persons galaces;[128] other shells are rough, as the limnostrea,[129] pinnæ, some kinds of conchæ, and the whelk; and of these some are marked with ridges, as the pecten and a kind of concha, others are without ridges, as the pinna and another species of concha. They also differ in thickness and thinness, both in the whole shell and in certain parts of the shell, as about the edges, for in some the edges are thin, as the mya; others are thick-edged, as the limnostrea. [128] Chama, L. [129] Ostrea edulis. 4. Some of them are capable of motion, as the pecten, for some persons say that the pectens can fly, for that they sometimes leap out of the instrument by which they are taken. Others, as the pinna, cannot move from the point of attachment; all the turbinated shells can move and crawl; the lepas (patella) also feeds by going from place to place. It is common to all those with hard shells to have them smooth in the inside. 5. Both in univalves and bivalves the fleshy part is united to the shell, so that it can only be separated by force; it is more easily separated from the turbinated shells; it is a characteristic of all these shells, that the base of the shell has the helix directed from the head. All of them from their birth have an operculum; all the turbinated testacea are right-handed, and move, not in the direction of the helix, but the contrary way. 6. The external parts of these creatures are thus distinguished; the nature of their internal structure is similar in all, especially in the turbinated animals, for they differ in size and in the relations of excess, the univalves and bivalves do not exhibit many differences. Most of them have but few distinctive marks from each other, but they differ more from the immovable creatures. This will be more evident from the following considerations. In nature they are all alike, the difference, as before said, is in excess; for in larger species the parts are more conspicuous, and less so in those that are smaller. They differ also in hardness and softness, and such like affections. 7. For all have on the outside of the shell, in the mouth, a hard piece of flesh, some more, some less; from the middle of this are the head and the two horns; these are large in larger species, in the little ones they are very small. The head is protruded in the same manner in all of them, and when the creature is alarmed it is again retracted; some have a mouth and teeth, as the snail, which has small, sharp, and smooth teeth.